The present invention relates to an optical probe to be incorporated in a scanning near field optical microscope (SNOM) which is used for the morphological observation of the specimen or for the measurement of optical properties in a nanometer region of a solid surface by irradiating light or by optical excitation of the surface of an object to be measured. The present invention also relates to a method for producing the optical probe and to a scanning near field optical microscope using the same.
Aperture type optical probes produced by coating a metal around sharpened glass capillaries or optical fibers have been reported heretofore. The advancements in micro-machining technology have enabled the production of probes having a very acute front end portion and have led to the realization of a scanning near field optical microscope (sometimes referred to simply hereinafter as an xe2x80x9cSNOMxe2x80x9d) capable of optical images with superior resolution power as compared with the conventional optical microscopes. Furthermore, the improvement in the precision of semiconductor processes has made it possible to produce optical waveguide optical probe chips of cantilever type.
However, in the case of the aperture type optical probes obtained to the present time, the light provided from an external light source must be introduced to a minute region of the specimen through the waveguide of the probe itself. To realize this, an operation of coupling the light to the fiber is required, and, moreover, an extremely complicated operation is necessary to handle a fine and brittle fiber amounting to a length of about 1 meter. Furthermore, in the case of optical waveguide optical probe chips of a cantilever size produced by a semiconductor process, the coupling of the light emitted from an external light source is so difficult that light leakage negatively influences the observation under the SNOM. In addition to this, the probe itself yields a low through put of light that an expensive laser with high power is required to be used as the external light source. Moreover, depending on the object, there are cases in which light with different wavelength is necessary, and in such cases, plural laser radiation sources capable of emitting the desired wavelengths must be provided separately. Because a laser radiation source emits a highly monochromatic light, it is necessary to convert the wavelength using a non-linear optical effect in order to comply with the requirement for a variety of light ranging over a wide wavelength; however, since the fiber itself poses limits in the wavelength of the transmitting light, the measurement of absorbance of the specimen under the SNOM, for example, is found unfeasible. In U.S. Pat. Nos. 5,546,223 and 5,105,305 by R. E. Betzig and U.S. Pat. No. 5,479,024 by Hillner are disclosed methods comprising placing a light emitting material in the front end of the probe chip and allowing the light-emitting material to emit light to use as a light source. However, these methods still required an external light source because photo excitation by using an external light source was used to realize the light emission of the light-emitting material.
According to a first aspect of the present invention, there is provided an optical probe having a field emitting function inside the optical probe as a means for solving the problem described above, and the optical probe is constructed by a thin film light emitting device. The optical probe can be produced by chemical etching, sputtering, vacuum evaporation, spin coating, and dipping. Thus, an inexpensive SNOM system is made feasible by using the optical probe of the present invention because an external light source and the optical system for light conversion can be eliminated this invention. Furthermore, an SNOM which can be used in the measurement of absorbance is made possible because the probes themselves are provided with the light emitting function. This enables the emission of plural lights differing in wavelength or a single light with a wide wavelength by properly selecting the organic material to be used therein. It is also possible to amplify the intensity of light by using a quartz rod, an optical fiber, or a hollow fiber for the core and thereby providing a cavity using the optical waveguide thereof. Since the optical probe itself emits light by simply applying voltage to the electrodes provided to the optical probe, the operability of the optical probe can be greatly improved; that is, it can be easily maneuvered in a manner similar to that of a cantilever used in AFM.